Radio communication system

ABSTRACT

A radio communication system has means for enabling the control channels associated with a data channel transmitting packet data with a low duty cycle to be switched to a dormant state or interrupted entirely. This reduces the excessive overhead that such control channels represent on a data channel only using a small proportion of the available channel capacity.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of prior application Ser. No. 09/478,468 filedJan. 6, 2000 now U.S. Pat. No. 6,668,168.

The present invention relates to a radio communication system andfurther relates to primary and secondary stations for use in such asystem and to a method of operating such a system. While the presentspecification describes a system with particular reference to theemerging Universal Mobile Telecommunication System (UMTS), it is to beunderstood that such techniques are equally applicable to use in othermobile radio systems.

There are two basic types of communication required between a BaseStation (BS) and a Mobile Station (MS) in a radio communication system.The first is user traffic, for example speech or packet data. The secondis control information, required to set and monitor various parametersof the transmission channel to enable the BS and MS to exchange therequired user traffic.

In one embodiment of UMTS control channels are maintained in bothdirections between BS and MS once a connection has been established.This is only a relatively small overhead when speech data is beingtransmitted. However, in the case of packet data having a low duty cycle(i.e. intermittent transmission of packets using only a small proportionof the available channel capacity), the maintenance of bidirectionalcontrol channels represents a significant overhead.

An object of the present invention is to reduce the overhead imposed bymaintaining control channels for a connection transferring data having alow duty cycle.

According to a first aspect of the present invention there is provided aradio communication system comprising a primary station and a pluralityof secondary stations, the system having a communication channel betweenthe primary station and a secondary station, the channel comprising anuplink and a downlink control channel for the transmission of controlinformation, and a data channel for the transmission of data packets,characterised in that the primary and secondary stations have trafficreduction means for reducing traffic in the uplink and downlink controlchannels, and control means for activating the traffic reduction means.

According to a second aspect of the present invention there is provideda primary station for use in a radio communication system having acommunication channel between the primary station and a secondarystation, the channel comprising an uplink and a downlink control channelfor the transmission of control information, and a data channel for thetransmission of data packets, characterised in that traffic reductionmeans are provided for reducing traffic in the downlink control channel,and control means are provided for activating the traffic reductionmeans.

According to a third aspect of the present invention there is provided asecondary station for use in a radio communication system having acommunication channel between the secondary station and a primarystation, the channel comprising an uplink and a downlink control channelfor the transmission of control information, and a data channel for thetransmission of data packets, characterised in that traffic reductionmeans are provided for reducing traffic in the uplink control channel,and control means are provided for activating the traffic reductionmeans.

According to a fourth aspect of the present invention there is provideda method of operating a radio communication system comprising a primarystation and a plurality of secondary stations, the system having acommunication channel between the primary station and a secondarystation, the channel comprising an uplink and a downlink control channelfor the transmission of control information, and a data channel for thetransmission of data packets, characterised by the primary and secondarystations being able to reduce traffic in the uplink and downlink controlchannels.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, wherein:

FIG. 1 is a block schematic diagram of a radio communication system;

FIG. 2 illustrates a conventional scheme for the transmission of packetdata;

FIG. 3 illustrates a scheme in accordance with the present invention forthe transmission of packet data with a control channel having a dormantstate in which traffic on the control channel is reduced;

FIG. 4 illustrates a scheme in accordance with the present invention forthe transmission of packet data with an interruptible control channel;and

FIG. 5 is a flow chart illustrating a method in accordance with thepresent invention for utilising a control channel having dormant andinterrupted states.

In the drawings the same reference numerals have been used to indicatecorresponding features.

Referring to FIG. 1, a radio communication system which can operate in afrequency division duplex mode comprises a primary station (BS) 100 anda plurality of secondary stations (MS) 110. The BS 100 comprises amicrocontroller (μC) 102, transceiver means (Tx/Rx) 104 connected toradio transmission means 106, power control means (PC) 107 for alteringthe transmitted power level, and connection means 108 for connection tothe PSTN or other suitable network. Each MS 110 comprises amicrocontroller (μC) 112, transceiver means (Tx/Rx) 114 connected toradio transmission means 116, and power control means (PC) 118 foraltering the transmitted power level. Communication from BS 100 to MS110 takes place on a downlink frequency channel 122, while communicationfrom MS 110 to BS 100 takes place on an uplink frequency channel 124.

Embodiments of the present invention will be described using spreadspectrum Code Division Multiple Access (CDMA) techniques, as used forexample in UMTS embodiments. However, it should be understood that theinvention is not limited to use in CDMA systems.

One UMTS embodiment, the frequency division duplex mode, uses the schemeof FIG. 2 for a communication link between MS 110 and BS 100. The linkis initiated by the MS 110 transmitting a request 202 (REQ) forresources on the uplink channel 124. If it receives the request and hasavailable resources, the BS 100 transmits an acknowledgement 204 (ACK)on the downlink channel 122 providing the necessary information forcommunication to be established. After the acknowledgement 204 has beensent, two control channels (CON) are established, an uplink controlchannel 206 and a downlink control channel 208.

The control channels 206, 208 include pilot, power control and rateinformation. The pilot information is primarily provided to allow thereceiver to estimate the channel impulse response, in order to optimisedetection of the received data (i.e. other information in the controlchannel and a data packet if present). Power control of the uplinkchannel 124 is required so that the BS 100 receives signals fromdifferent MS 110 at approximately the same power level, while minimisingthe transmission power required by each MS 110. Power control of thedownlink channel 122 is required so that the MS 110 receives signalsfrom the BS 100 with a low error rate while minimising transmissionpower, to reduce interference with other cells and radio systems. Rateinformation provides details of the rate and transmission format ofdata, to enable the transceivers 104, 114 to be appropriatelyconfigured.

In the scenario illustrated in FIG. 2, the MS 110 transmits data packets210 (DAT) with lengthy periods between them, during which periodssignificant resources are being used on both the uplink 124 and downlink122 channels just to maintain control channels 206, 208. Hence the totalsystem capacity for the transmission of user traffic is reduced. Atypical duration for a data packet 210 in a UMTS system would be oneframe (10 ms).

An improved scheme for low duty cycle traffic, in accordance with thepresent invention, is shown in FIG. 3. Initiation of the link proceedsin the same manner as described above in relation to FIG. 2. Inaddition, the uplink 206 and downlink 208 control channels enter adormant state 302, 304 (DOR) between transmission of data packets 210.The dormant state might for example be entered after a time-out period,or immediately after transmission of a data packet if the transmittingstation can determine that no further data packets are currentlyavailable for transmission

In this dormant state no rate information is transmitted (or possibly areduced amount of rate information), and the power control rate isreduced, for example only transmitting power control information in oneslot out of N. This will produce a larger error in power control thanwith continuous transmission. Increasing the power control step sizewill partly address the problem, and other methods described in ourco-pending application GB 9900910.2 (our reference PHB 34314) can beapplied to improve matters further. Pilot information need only betransmitted when power control bits are being transmitted. By thismethod the overhead of maintaining control channels 206, 208 issignificantly reduced with a corresponding increase in overall systemcapacity. If the packet duty cycle is 10%, the overhead will be reducedby a factor of about two if N=2.

When the MS 110 has another data packet 210 to send it transmits are-activation request 306 as part of the dormant channel (or usesalternative signalling means such as a dedicated fast signallingchannel). Once the BS 100 has transmitted an acknowledgement 308 of there-activation request 306, the normal control channels 206, 208 arere-activated and the data packet 210 can be transmitted.

A scheme similar to that described above could also be used forcontinuous transmission of data. Consider for example a channel that isnormally used for high data rate transmissions. If this channel becomesused for transmission of data at a low data rate, the optimum rate oftransmission of power control information might also be reduced. Hence,while the data continues at a low rate, the quantity of controlinformation could be reduced in the same way as above.

Reducing the amount of power control information transmitted usuallyrequires the data channel to be transmitted at a higher power tomaintain the same quality of service. However, for a low data ratetransmission the increased power required for the data transmissioncould be more than offset by the reduction in the transmsission ofcontrol information.

An alternative scheme, in which the control channel is interruptedcompletely, for example after a suitable time-out period, is shown inFIG. 4. This scheme completely removes the overhead of the controlchannels 206, 208 between transmission of data packets 210. However,re-activation of the control channels 206, 208 can be done with specificsignalling. This is illustrated as a re-activation request 402 from theMS 110, transmitted for example on a dedicated fast signalling channel,followed by an acknowledgement 404 by the BS, after which the controlchannels 206, 208 are re-established and data packets 210 can betransmitted.

In practice it may not be necessary for there to be an explicitacknowledgement 404 for reactivation of the control channels 206, 208.For example, if the downlink control channel 208 uses the same spreadingcodes after interruption as before interruption, the BS 100 couldreactivate the channel by simply starting to transmit it again, whichtransmission would be detected by the MS 110 which could then begintransmission of the uplink control channel 206. Similarly, the MS 110could request re-activation by starting transmission of the uplinkcontrol channel 206. The same approach could also be used forre-activation from the dormant state.

If the communication system employs CDMA techniques, the controlchannels 206, 208 may be transmitted with a different spreading factorto the data transmissions 210. In this case satisfactory qualityestimates for use in power control might not be able to be derivedaccurately from the control channels 206, 208 alone, and completeinterruption of them would not have any additional impact on powercontrol.

The use of dormant and interrupted control channels may be combined. Amethod of using dormant and interrupted control channels for thetransmission of data packets is shown as a flow chart in FIG. 5. Themethod starts at 502 with the establishment of a communication linkbetween the MS 110 and BS 100. The MS 110 then determines at 504 whetherit has any data packets 210 to transmit. If it has they are transmitted506. When no more data packets 210 remain to be transmitted the MS 110determines at 508 whether a first time-out period since transmission ofthe last data packet 210 has been reached. If it has, the controlchannels 206, 208 enter their dormant state at 510.

The MS 110 checks, at 512, whether any data packets 210 are waiting fortransmission. If any are waiting, the control channels 206, 208 arereestablished at 516, and the data packets are transmitted 506. If thereare no waiting data packets 210, the MS 110 determines at 514 whether asecond time-out period since transmission of the last data packet 210has been reached. If it has, transmission of the dormant controlchannels 302, 304 is interrupted. The MS 110 then waits at 520 until oneor more data packets 210 are waiting for transmission, when itre-establishes the control channels at 516 and transmits the datapackets at 506.

The time-out periods should preferably be chosen to be short enough tosave resources, but not so short that frequent transitions betweennormal, dormant and interrupted states are needed. A suitable choicecould be between 5 and 10 frames for the first time-out 508, and asimilar period for the second time-out 514.

Although the description above has examined data transmission on theuplink channel 124, the techniques are equally applicable to datatransmission on the downlink channel 122 or to bidirectionaltransmissions. In this latter case the preferred solution would be toreset the time-out period on packet transmission in either the uplink ordownlink data channel. Although in principle the control channel state(i.e. normal, dormant or interrupted) could be different in the uplink124 and downlink 122 channels, there seems to be no advantage inallowing this.

Further, the present invention is not restricted to use in a frequencydivision duplex system. All that is required is for uplink and downlinkcommunication paths to be available. It could, for example, be used in atime division duplex system, although the power control rate in such asystem would normally be limited to once per transmission burst.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in radio communication systemsand component parts thereof, and which may be used instead of or inaddition to features already described herein.

In the present specification and claims the word “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

1. A radio communication system comprising: a primary station, aplurality of secondary stations, a communication channel between theprimary station and a secondary station, the communication channelcomprising an uplink control channel and a downlink control channel forthe transmission of control information between the primary andsecondary stations, and a data channel for the transmission of datapackets, each of the primary and secondary stations having trafficreduction means for reducing traffic in the uplink and downlink controlchannels, and control means for activating the traffic reduction means,the control means activating the traffic reduction means after a firstdata transmission on the data channel to cause the uplink and downlinkcontrol channels to enter into a dormant state during which controlinformation is transmitted on the uplink and downlink control channels,the traffic reduction means being arranged to cause transmission of areduced amount of control information on the uplink and downlink controlchannels while the uplink and downlink control channels are in thedormant state than the amount of control information transmitted on theuplink and downlink control channels during transmission of data packetson the data channel.
 2. A system as claimed in claim 1, wherein thecontrol means activates the traffic reduction means after apredetermined period has passed without transmission of a data packet onthe data channel after the first data transmission.
 3. A system asclaimed in claim 2, wherein the traffic reduction means are arranged totransmit control information on the uplink and downlink control channelsin allocated time slots and operate to transmit control information inone out of every N available slots when the uplink and downlink controlchannels are in the dormant state, where N is an integer greater than 1.4. A system as claimed in claim 1, wherein the control means activatesthe traffic reduction means immediately after the first datatransmission when the control means determines that no additional datatransmission is currently available for transmission.
 5. A system asclaimed in claim 4, wherein the traffic reduction means are arranged totransmit control information on the uplink and downlink control channelsin allocated time slots and operate to transmit control information inone out of every N available slots when the uplink and downlink controlchannels are in the dormant state, where N is an greater than
 1. 6. Asystem as claimed in claim 1, wherein the traffic reduction means arearranged to transmit control information on the uplink and downlinkcontrol channels in allocated time slots and operate to transmit controlinformation in one out of every N available slots when the uplink anddownlink control channels are in the dormant state, where N is aninteger greater than
 1. 7. A system as claimed in claim 1, wherein thetraffic reduction means are arranged to interrupt transmission of theuplink and downlink control channels after a time period has elapsedwithout transmission of a data packet on the data channel while theuplink and downlink control channels are in the dormant state.
 8. Aprimary station for use in a radio communication system having acommunication channel between the primary station and a secondarystation, the communication channel comprising an uplink and a downlinkcontrol channel for the transmission of control information between theprimary station and the secondary station, and a data channel for thetransmission of data packets, the primary station comprising: trafficreduction means for reducing traffic in the downlink control channel,and control means for activating the traffic reduction means, thecontrol means activating the traffic reduction means after a first datatransmission on the data channel to cause the uplink and downlinkcontrol channels to enter into a dormant state during which controlinformation is transmitted on the uplink and downlink control channels,the traffic reduction means being arranged to cause transmission of areduced amount of control information on the uplink and downlink controlchannels while the uplink and downlink control channels are in thedormant state than the amount of control information transmitted on theuplink and downlink control channels during transmission of data packetson the data channel.
 9. A primary station as claimed in claim 8, whereinthe control means activates the traffic reduction means after apredetermined period has passed without transmission of a data packet onthe data channel after the first data transmission.
 10. A primarystation as claimed in claim 9, wherein the traffic reduction means arearranged to transmit control information on the uplink and downlinkcontrol channels in allocated time slots and operate to transmit controlinformation in one out of every N available slots when the uplink anddownlink control channels are in the dormant state, where N is aninteger greater than
 1. 11. A primary station as claimed in claim 8,wherein the traffic reduction means are arranged to transmit controlinformation on the uplink and downlink control channels in allocatedtime slots and operate to transmit control information in one out ofevery N available slots when the uplink and downlink control channelsare in the dormant state, where N is an integer greater than
 1. 12. Asecondary station for use in a radio communication system having acommunication channel between the secondary station and a primarystation, the communication channel comprising an uplink and a downlinkcontrol channel for the transmission of control information between theprimary station and the secondary station, and a data channel for thetransmission of data packets, the secondary station comprising: trafficreduction means for reducing traffic in the uplink control channel, andcontrol means for activating the traffic reduction means, the controlmeans activating the traffic reduction means after a first datatransmission on the data channel to cause the uplink and downlinkcontrol channels to enter into a dormant state during which controlinformation is transmitted on the uplink and downlink control channels,the traffic reduction means being arranged to cause transmission of areduced amount of control information on the uplink and downlink controlchannels while the uplink and downlink control channels are in thedormant state than the amount of control information transmitted on theup link and downlink control channels during transmission of datapackets on the data channel.
 13. A secondary station as claimed in claim12, wherein the control means activates the traffic reduction meansafter a predetermined period has passed without transmission of a datapacket on the data channel after the first data transmission.
 14. Asecondary station as claimed in claim 13, wherein the traffic reductionmeans are arranged to transmit control information in allocated timeslots and operate to transmit control information in one out of every Navailable slots when the uplink and downlink control channels are in thedormant state, where N is an integer greater than
 1. 15. A secondarystation as claimed in claim 12, wherein the traffic reduction means arearranged to transmit control information in allocated time slots andoperate to transmit control information in one out of every N availableslots when the uplink and downlink control channels are in the dormantstate, where N is an integer greater than
 1. 16. A method of operating aradio communication system comprising a primary station and a pluralityof secondary stations, the system having a communication channel betweenthe primary station and a secondary station, the communication channelcomprising an uplink and a downlink control channel for the transmissionof control information between the primary station and the secondarystation, and a data channel for the transmission of data packets, themethod comprising: entering the uplink and downlink control channel intoa dormant state after a first data transmission on the data channel;transmitting control information on the uplink and downlink controlchannels during transmission of data packets; and transmitting an amountof control information the uplink and downlink control channels whilethe uplink and downlink control channels are in the dormant state whichis less than the amount of control information transmitted on the uplinkand downlink control channels during transmission of data packets on thedata channel.
 17. A method as claimed in claim 16, wherein the uplinkand downlink control channels enter into the dormant state and thereduction in transmission of control information on the uplink anddownlink control channels is initiated after a predetermined period haspassed without transmission of a data packet on the data channel afterthe first data transmission.
 18. A method as claimed in claim 17,wherein control information is transmitted in allocated time slots, andthe reduction in the amount of control information transmitted while theuplink and downlink control channels are in the dormant state comparedto that transmitted during transmission of data packets is realized bytransmitting control information in one out of every N available slotswhile the uplink and downlink control channels are in the dormant state,where N is an integer greater than
 1. 19. A method as claimed in claim16, wherein control information is transmitted in allocated time slots,and the reduction in the amount of control information transmitted whilethe uplink and downlink control channels are in the dormant statecompared to that transmitted during transmission of data packets isrealized by transmitting control information in one out of every Navailable slots while the uplink and downlink control channels are inthe dormant state, where N is an integer greater than
 1. 20. A method asclaimed in claim 16, wherein the uplink and downlink control channelsenter into the dormant state and the reduction in transmission ofcontrol information on the uplink and downlink control channels isinitiated immediately after the first data transmission when the controlmeans determines that no additional data transmission is currentlyavailable for transmission.